This invention relates in general to gas testing devices and in particular to a gas monitor with an indicator band and a foretube.
In prior art gas monitors, indicator bands are provided which are continuously or stepwise moved to be brought into contact with the gas to be monitored. The gas sample is fed in by a pump. Some of the gases to be measured cannot be determined with the indicator band directly. They do not react with the impregnating agent. Gases of this kind, for example vinyl chloride, must be chemically converted before they can be measured. The product thereby obtained, such as chlorine in this example, then reacts on the indicator band. The conversion takes place in a foretube, which is designed as a kind of a conventional gas detector tube. Within the gas monitor, such a foretube is disposed in the path of the sample upstream of the indicator band. The substance effecting the chemical conversion and received in the foretube determines the reaction time and thus the service time of the indicator band.
In a prior art device for monitoring fluids, an indicator band is mechanically advanced to bring it into contact with the fluid to be tested. A pump forces the fluid through the indicator band. The band indicates the noxious compounds of the fluid by changing color. However, no indicator is known for vinyl chloride. That is why a preliminary treatment of the fluid is necessary. To this end, a tube for preliminary treatment, or foretube, is provided upstream of the fluid inlet of the monitor, which is filled with a known oxidizing agent. While passing therethrough, vinyl chloride is oxidized to chlorine and indicators for chlorine are known (German OS No. 26 15 375). If a continuous monitoring is involved, the problem arises that even though the advance of the indicator band makes sure that always a fresh indicating portion is available for the test, the oxidizing agent in the foretube has only a limited capacity. This either undesirably reduces the service time, or requires larger sizes of foretubes with the result of unhandy dimensions and of an increased resistance to flow.